Some known capacitive-based tactile sensors use the relative change in distance d between two conductive plates as a way to measure the applied pressure pa, namely:
                                                        p              a                        ∝            C                    =                                    ɛ              r                        ⁢                          ɛ              0                        ⁢                          A              d                                      ,                            (        1        )            
where C is the capacitance value, ∈r and ∈0 are respectively the relative static permittivity and the vacuum permittivity and A is the overlap area of the plates. One element that often limits the sensitivity of these sensors is the mechanical response of the non-conductive material used between the electrodes and the ground plane, that serves as a dielectric as well as a spring-like element. It has been known to use a plain layer of soft polymer such as silicone [1] to create a soft dielectric (i.e., a non-rigid or resilient) that will deform under applied stress.
However, silicone and many other polymers used for such purpose may be considered as being incompressible. Hence, for a wide range of applied pressure, their volume will remain unchanged. In reaction to a compressive force on a specific area, some region of the dielectric will have to expand. This behavior may lead to inadequate compliance of the soft material as well as a relatively slow recovery time once pressure is released, two shortcomings that may result in hysteresis and in a relatively low sensor sensitivity.
To circumvent these drawbacks, silicone foam [2] or urethane foam [3] have been used as a dielectric for capacitive-based tactile sensor. Although, the volume of a foam under pressure can change, thus leading to a pressure sensor with increased sensitivity, regular foam is also subject to hysteresis. Ulmen et al. [4] have thus suggested using a closed cell polyurethane foam. The ratio of stiffness to damping in a foam with a closed cell structure is higher due to the trapped air that acts as spring-like elements. The nonlinearity of the response may thus be reduced. However, because of this structure, the stiffness of these foams is typically relatively high and their permittivity relatively low, thus leading to an average sensitivity. In order to increase the sensitivity and decrease the hysteresis, Mannsfeld et al [5] made a significant breakthrough by creating a microstuctured dielectric made out of polydimethylsiloxane (PDMS). By creating microfeatures in a PDMS layer using soft lithography, a material has been created with room for silicone to expand locally. Consequently, the apparent stiffness of the dielectric has been lowered and hysteresis has been reduced. However, the manufacturing of such a sensor, due to the size of the features, is based on soft lithography. Soft lithography is a time-consuming process typically used for making micro-electro-mechanical systems (MEMS) that requires a significant amount of specialized equipments.